Epidural analgesia is a method for managing pain during labor and certain surgical procedures. It involves delivering medication into a specific area of the back to numb the lower body. This process is initiated with a specialized hollow needle, which serves as the tool for accessing the target location. The design of this needle is specific to the requirements of the procedure.
Anatomy of the Epidural Needle
The needle for an epidural is distinct from those for standard injections, being both longer and thicker. Its size, 16 to 18 gauge, is needed to navigate through skin, fat, and dense ligaments to reach the epidural space. This is a potential space, meaning its surfaces are normally close together, and the needle creates a path to it. The needle’s length, around 8 cm, has markings at 1 cm intervals to help the anesthesiologist judge insertion depth.
A component of the epidural needle is the stylet, a solid, removable inner wire. The stylet provides rigidity during insertion and fills the needle’s hollow bore, preventing tissue from being plugged into the needle as it advances. Once the needle is positioned, the stylet is removed, ensuring a clear channel for placing a catheter.
The most common type is the Tuohy needle, which features a curved, blunt tip. This curve, between 15 and 30 degrees, is designed to guide a flexible catheter into the epidural space without puncturing the dura mater. The blunt, angled tip helps direct the catheter sideways into the epidural space.
The Epidural Placement Procedure
The process begins with the patient positioned either sitting and leaning forward or lying on their side with knees drawn up. This posture curves the back, opening the spaces between the vertebrae and making access easier. The anesthesiologist cleans the skin with an antiseptic solution to minimize the risk of infection.
Before the epidural needle is inserted, a local anesthetic is injected into the skin and deeper tissues at the site. This numbs the area, reducing the sensation of pain when the larger epidural needle is introduced. Patients usually report feeling a brief sting from this medication, followed by pressure during the main procedure.
With the area numbed, the anesthesiologist inserts the epidural needle through the skin and between the vertebrae. The needle is advanced through several layers of tissue, including the supraspinous and interspinous ligaments, and finally the ligamentum flavum. This final ligament has a dense consistency that provides a noticeable increase in resistance.
To confirm the needle has reached the correct destination, clinicians use a “loss of resistance” technique. A syringe filled with saline or air is attached to the needle hub, and the anesthesiologist applies constant pressure to the plunger while advancing the needle. Upon entering the epidural space, the resistance from the ligamentum flavum suddenly gives way, allowing the plunger to move forward easily. This change signals that the needle tip is correctly placed.
The Function of the Epidural Catheter
Once the needle’s position is confirmed, a thin, flexible tube called an epidural catheter is threaded through the hollow needle. The catheter, which has markings to indicate its length, is advanced a few centimeters into the epidural space.
After the catheter is situated, the epidural needle is withdrawn over it. The large, rigid needle does not remain in the patient’s back; only the soft, pliable catheter is left in place. It is secured to the back with adhesive tape and its flexibility allows the patient to move without discomfort.
The catheter serves as a conduit for delivering pain-relieving medication. Anesthetic and analgesic drugs can be administered through it in continuous infusions or intermittent doses for as long as needed. This method provides sustained relief by bathing the nerve roots in the epidural space with medication.
Risks Associated with Needle Insertion
The primary risk from the needle insertion is an accidental dural puncture, or “wet tap.” This occurs if the needle advances too far, piercing the dura mater, the membrane containing cerebrospinal fluid (CSF). The incidence of a wet tap is estimated to be between 0.5% and 1.5% of epidural procedures.
When a wet tap happens, the leakage of cerebrospinal fluid can lower the pressure within the skull, leading to a post-dural puncture headache (PDPH). These headaches are positional, worsening when sitting or standing and improving when lying flat. They can be accompanied by neck stiffness, nausea, and visual disturbances, and while many resolve with conservative treatment, some require further intervention.
Other, less common risks include localized pain or bruising at the injection site. Bleeding in the epidural space, known as an epidural hematoma, is a rare but serious complication that can compress the spinal cord. Infection at the insertion site is also a possibility.